Anodizing electrolyte and process



United States Patent 3 280,013 ANQDIZING ELECTROLYTE AND PROCES? George Economy, Delmont, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Filed June 2, 1964, Ser. No. 372,088 5 Claims. (Cl. 20458) This invention relates to the anodizing of aluminum, by which generally is meant the metal aluminum, pure and impure and the various aluminum base alloys. The object of the invention is to provide an anodizing electrolyte and process having multi-purpose utility and approaching to a large degree the requirements hereinafter defined.

Anodic .oXide coating of aluminum, which involves the use of an aqueous electrolyte in which the aluminum is made anode, is a useful technique widely used to provide an aluminum surface or a portion thereof with an oxide coating serving one or more useful functions such as protective, decorative, adsorptive or the like. The requirements of such a process may be widely variant in object from one aluminum product to another. In some cases a clear or substantially colorless coating may be desired. In other cases a protective coating with no emphasis on appearance may be desired. In still other cases, a coating substantially colorless as produced but receptive to a dye or pigment for decorative purposes may be desired. Further, in some cases, particularly for outdoor exposure, a coating having some lightfast color or tint as produced may be desired. The processor who produces such anodic coatings has need for a process and electrolyte suited to such multi-purpose employment, in the sense that the same can advantageously be used for one or more of these purposes, that present no real problem of voltage rise beyond the capacity of equipment when operated at substantial current density, and that are capable of being operated over a relatively wide range of temperature. The present invention provides an anodizing process and an electrolyte therefor which meet these conditions to a substantial degree.

The process of this invention employs an electrolyte containing succinic acid, a non-acid hydrogen of which is substituted with the group SO H. Such a compound has been described in the literature, of. Beilstein, volume 4, page 25; Beilstein, 2nd Supplement, volume 4, pages 537-538; and Chemical Abstracts, volume 21, page 3351. Various ways of preparing the compound may be employed. The electrolyte of the invention also contain sulfuric acid and water. The amount of sulfuric acid and the proportioning of the sulfuric acid content to the substituted succinic acid content are, as hereinafter explained, desirably governed by specific purposes for which particular anodic coatings are intended. The sulfuric acid content in a given electrolyte may be adjusted, for example, either by addition of sulfuric acid or removal of sulfuric acid, in the latter case by employment of a sulfate precipitant such as barium hydroxide or other precipitant, preferably one the use of which does not introduce into the electrolyte soluble metal which might be detrimental to the operation, as hereinafter noted.

The electrolyte of the invention is essentially substituted succinic acid, sulfuric acid, and water. The substituted succinic acid content of the electrolyte may be introduced directly by the use of the substituted succini acid. If a salt of'the substituted succinic acid is used, e.g., a sodium neutralized substituted succinic acid compound, the same should be ion-exchanged to make available in the solution the desired free substituted succinic acid content. The amount of the substituted succinic acid to be used may range from a practical coating forming minimum, such as so called grays and blacks.

3,230,013 Patented Oct. 18, 1966 ice about 5 grams per liter, to any higher amount desired in the aqueous electrolyte, but generally no more is used than is advantageous so as to avoid undue expense arising from the employment of this acid. In the practice of the invention, amounts greater than about 125 grams per liter do not presently appear to afford special advantage, and to reduce costs a convenient operating range for many purposes is about 35 to about grams per liter. As hereafter exemplified, there are in the practice of this invention some concentrations of the substituted succinic acid which are preferable for specific purposes. Where some change in concentration of the substituted succinic acid is to be made, it may be achieved by dilution with water, or conversely by circulating a portion or all of the electrolyte through a rapid evaporator to remove a portion of the water. This acid is sufiiciently stable. to tolerate concentration by evaporation.

The sulfuric acid content of the electrolyte may be introduced directly by the use of sulfuric acid, but a water soluble sulfate donor such as a metal, ammonium, or organic sulfate or bisulfate may be used to introduce the sulfuric acid content if desired. In the latter cases the sulfuric acid content may be regarded as provided by the sulfate donor and some of the acid hydrogen from the substituted succinic acid present, the effective amount of free substituted succinic acid in such cases thereby being reduced somewhat, unless restored by ion-exchange in accordance with usual ion-exchcange practices. The

amount of sulfuric acid to be used may range from a prac-' tical coating forming minimum, such as about /2 gram per liter, to any higher amount desired in the aqueous electrolyte. Generally, in the practice of the invention, amounts greater than about 250 grams per liter do not appear to afford special advantage. A convenient operating range for some purposes may be a range above about 15 and up to about 200 grams per liter, while for other purposes a relatively low range such as about 2 to about 12.5 grams per liter is preferred. Adjustment of the sulfuric acid content may be made as hereinbefore described, i.e., by addition of sulfuric acid or precipitation of insoluble sulfate.

In the use of this invention for producing coatings which are tinted or colored as produced, it should be realized that the tints or colors readily obtained are usually in a range of yellows and browns (which might be termed by an observer as light to dark bronzes), and the Herein these terms as to tint or color convey no more than an approximation since the exact characterization of a tint or color is, as to any given observer, a matter of language and degree of emphasis. The intensity of the tints or colors will generally depend upon the anodizing time, i.e., the coating thickness (which is normally a function of anodizing time), the particular composition of the electrolyte, and the particular composition of the aluminum and its prior fabrication and thermal history. Every aluminum composition or alloy has some limitation as to the tint shade or color of anodic coating which can readily be produced on it by the practice of this invention. Usually when operating in a range of coating thicknesses of 0.1 to 2 mils, for example, satisfactory shades of various depths in the yellows, browns, grays and blacks can be imparted to a variety of aluminum compositions and alloys by use of a solution in which sulfuric acid is present in a relatively small amount as hereinafter described. Generally, aluminum of what is currently termed commercial purity will not readily produce in the indicated coating thicknesses, tints or shades beyond yellows or golds to browns (or bronzes), but alloys of commercial purity aluminum with small amounts of manganese, or magnesium silicide,

QJI or both, will more readily produce darker tints or shades, including grays and blacks.

For the production of light colored or substantially colorless anodic coatings on aluminum in accordance with the invention, electrolytes such as those of the following compositions may be used:

Electrolyte 2 may be considered as a representative example for the indicated purpose, but within the range of electrolyte indicated by these examples, such range being but indicative, commercially pure articles of aluminum may be made anode therein and provided with substantially colorless anodic oxide coatings. ample of one of many operating conditions that may be employed in such cases, the electrolyte may be maintained at about 75 F., and the anodizing current density may be maintained at 36 amperes per square foot, for example. In minutes a coating about 0.5 mil thick will be formed; with longer periods of anodizing, thicker coatings may be produced. On articles of other aluminum compositions and alloys, electrolytes of the composition range indicated generally produce coatings with approximately the same lack of color or shade of color that is obtained in coatings of corresponding thickness produced by conventional anodizing practices in percent sulfuric acid electrolytes. As previously indicated here-in, for light colored or substantially colorless coatings the sulfuric acid content of the electrolyte is preferably adjusted and maintained at a level above about 15 grams per liter.

For the production of anodic coatings having some distinctive color or tint, there may be employed in accordance with the invention, electrolytes of a composition range such as is generally indicated by the following compositions:

Composition (g./l.balance water) Example Electrolyte Substituted Suceinic Acid Sulfuric Acid It is desirable for such a purpose that the amount of sulfuric acid present in the electrolyte should be at least about /2 gram per liter, preferably about 2 to about 12.5 grams per liter, and that the weight ratio of sulfuric acid to the substituted succinic acid be no more than about 1 to 8. For example, if the substituted succinic acid c0ntent is 100 grams per liter or more, the sulfuric acid content preferably is no more than about 12.5 grams per liter. Electrolyte C may be considered as a preferred example, but with any of the above electrolytes various aluminum compositions and alloys, pure or impure, may be made anode therein and provided with uniform coatings that are tinted or exhibit color. As an example of suitable operating conditions, the electrolyte may be maintained at about 75 F., and the anodizing current may be maintained at 24 amperes per square foot. Under such As an ex-- conditions the initial anodizing voltage with Electrolyte C, for example, will be about 35 volts, rising as the coat ing builds up in thickness. On commercially pure aluminum 20 minute, 30 minute and 40 minute coatings require final voltages on the order of 50, 70 and volts respectively in Electrolyte C, and acquire light, medium and deep yellow shades of color, respectively. A commercial aluminum alloy containing about 1 percent Mg Si, for example, acquires a color shade ranging from light brown, medium brown, near black to black, after 20, 30, 40 and 50 minutes, respectively, under the stated conditions of anodizing in Electrolyte C. A commercial aluminum alloy containing 1.25 percent manganese acquires a dark gray coating in 30 minutes under similar conditions.

In general, electrolyte temperatures at or near room temperature may be employed, but in the process of this invention there is a considerable latitude or permissibility of temperature of operation. Lower temperatures such as 40 F. or higher temperatures such as 100 F. are tolerable. Temperatures in the range between about 60 and about F. will usually be found desirable in most commercial operations.

The current density may be maintained at a lower value such as about 12 amperes per square foot or even lower, or at a higher value such as about 64 amperes per square foot or even higher. For coating thickness and color or shade matching a constant current density may be preferred, but changing current density may also be used. For example, practices employing a constant initial current density and thereafter a constant terminal voltage allowing the current density to decay may be employed.

As noted, the color or shade of color of an anodic oxide coating produced in accordance with the invention usually depends on the coating thickness, all other things being equal. The anodizing time will therefore be chosen with reference to the thickness and/ or color desired. Decorative coatings for indoor use may be made in thicknesses on the order of 0.1 mil, while protective coatings will usually be at least 0.4 mil in thickness to meet current northern hemispheric specifications, and on the order of 0.7 mil or more in thickness for outdoor service in temperate climate. The coatings produced in accordance with the invention are adsorptive and thus may be sealed if desired. They may also be supplementally dyed or pigmented so as to color the same or modify the color as produced.

It has been observed that, during anodizing in electrolytes employed in accordance with the invention, aluminum is gradually dissolved in the electrolyte, usually at a rate of about 0.08 to 0.09 gram of aluminum per ampere hour of anodizing. The dissolution of aluminum will reduce the conductivity of the electrolyte, reduce the throwing power of the electrolyte, and affect other operating characteristics. For example, an electrolyte originally containing 70 grams per liter of the substituted succinic acid and 5 grams per liter of sulfuric acid has, after 1 gram per liter of aluminum is dissolved therein, a conductivity approximately that of an otherwise similar electrolyte containing about 20 grams per liter less of the substituted succinic acid and no dissolved aluminum. While ordinary chemical analysis methods have not revealed whether any particular aluminum-substituted-succinate complex is actually formed in the electrolyte, or in what ratio the substituted succinate ions and aluminum ions tend to combine or interact in the electrolyte, it does appear that, under anodizing conditions and for such purposes, there is a deactivation of some of the substituted succinic acid content by aluminum dissolved in the electrolyte and approximately according to a combining ratio of one molecular weight of the former to one hydrogen equivalent weight of the latter. Thus, the reduction in conductivity and other eifects upon operating characteristics of the electrolyte accompanying dissolution of aluminum therein indicate that the effective free substituted succinic acid content of the electrolyte may be considered as less than the total substituted succinate present by about 20 times the weight of dissolved aluminum.

Considerations such as the foregoing indicate the desirability of avoiding undue accumulation of aluminum in the electrolyte during anodizing. Additions of the substituted succinic acid to the electrolyte may, of course, be made as required to maintain a desired content of effective free substituted succinic acid in the electrolyte during a given period of use. Uniformity of electrolyte operating characteristics is better obtained, however, if only a low, substantially constant level of aluminum is permitted to accumulate and remain in the electrolyte. The aluminum content may be controlled by batch or continuous bypass ion-exchange procedures, is desirably limited to a level of not more than about 2.5 grams per liter, and preferably is maintained at a level between about 0.1 and about 1 gram per liter.

The aluminum articles treated in accordance with the invention may be made of aluminum alone, or may be aluminum coated or clad products, or any other products presenting aluminum surfaces for anodizing.

What is claimed is:

1. An electrolyte for anodizing aluminum, consisting essentially of at least about 35 grams per liter of succinic acid, a nonacid hydrogen of which is substituted with the group SO H,

about 2 to about 12.5 grams per liter of sulfuric acid,

and

water,

the weight ratio of the sulfuric acid to the substituted succinic acid being no more than about 1 to 8.

2. An electrolyte as set forth in claim 1 which contains not more than about 2.5 grams per liter of dissolved aluminum.

3. A process for anodizing aluminum, comprising making an aluminum article anode in an electrolyte consisting essentially of at least about 5 grams per liter of succinic acid, a non-acid hydrogen of which 6 is substituted with the group -SO H, at least about /2 gram per liter of sulfuric acid, and

water.

4. A process for anodizing aluminum, comprising making an aluminum article anode in an electrolyte consisting essentially of at least about 35 grams per liter of succinic acid, a non-acid hydrogen of which is substituted with the group -SO H, about 2 to about 12.5 grams per liter of sulfuric acid, and water,

the weight ratio of the sulfuric acid to the substituted succinic acid being no more than about 1 to 8, while the electrolyte is maintained at a temperature between about and about F., and

the current density is maintained between about 12 and and about 64 amperes per square foot,

for a time of treatment suflicient to produce an anodic oxide coating at least 0.1 mil thick,

whereby a tinted anodic oxide coating is produced on the surface of the aluminum article.

5. A method in accordance with claim 4 in which the amount of aluminum dissolved in the electrolyte is controlled at a level between about 0.1 and about 1 gram per liter.

References Cited by the Examiner UNITED STATES PATENTS Re. 25,566 4/1964 Deal et al. 204-58 2,052,575 9/1936 Lilienfeld 20458 X 2,233,785 3/1941 Korpiun 20458 2,963,409 12/ 1960 Ramirez 204-58 3,031,387 4/1962 Deal et al 20458 3,098,018 7/1963 Kissin et al 20458 X 3,143,485 8/1964 Kampert et al 20458 3,146,178 8/1964 Cochran et al 204-58 3,227,639 1/1966 Kampert 204-58 JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner. 

3. A PROCESS FOR ANODIZING ALUMINUM, COMPRISING MAKING AN ALUMINUM ARTICLE ANODE IN AN ELECTROLYTE CONSISTING ESSENTIALLY OF AT LEAST ABOUT 5 GRAMS PER LITER OF SUCCINIC ACID, A NON-ACID HYDROGEN OF WHICH IS SUBSTITUTED WITH THE GROUP -SO2H, AT LEAST ABOUT 1/2 GRAM PER LITER OF SULFURIC ACID, AND WATER. 